Data storage media are commonly used for storage and retrieval of data, and come in many forms, such as magnetic tape, magnetic disks, optical tape, optical disks, holographic disks, cards or tape, and the like. Magnetic tape media remains an economical medium for storing large amounts of data. In magnetic tape, data is typically stored as magnetic signals that are magnetically recorded on the medium surface. The data stored on the magnetic tape is often organized along “data tracks,” and read/write heads are positioned relative to the data tracks to write data to the tracks or read data from the tracks.
Servo patterns refer to signals or other recorded marks on the medium that are used for tracking purposes. In magnetic tape, the servo patterns are recorded in specialized tracks on the medium called “servo tracks” to provide reference points relative to the data tracks. A servo controller interprets detected servo patterns and generates position error signals. The position error signals are used to adjust the lateral distance of the read/write head relative to the data tracks so that the read/write head is properly positioned along the data tracks for effective reading and/or writing of the data to the data tracks. A plurality of servo tracks may be defined in a servo band. Most magnetic media include a plurality of servo bands, with data tracks being located between the servo bands. The data tracks located between the servo bands may be defined as a data band. Time-based servo techniques refer to servo techniques that make use of time variables. Time-based servo techniques are particularly effective for magnetic tape, which typically feeds past transducer heads at a constant velocity. For example, N-shaped servo markings, servo markings such as “<<< >>>” or “//// \\\\,” or the like, have been developed for time-based servo techniques. Such markings are typically formed in a servo track of the magnetic tape.
When time-based servo techniques are used, the time offset between detection of two or more servo marks can be translated into a position error signal, which defines a lateral distance of the transducer head relative to a data track. For example, given a constant velocity of magnetic tape formed with marking “/\”, the time between detection of “/” and “\” becomes larger when the servo head is positioned towards the bottom of marking “/\” and smaller if the servo head is positioned towards the top of marking “/\”. Given a constant velocity of magnetic tape, a defined time period between detected servo signals may correspond to a center of marking “/\”. By locating the center of marking “/\”, a known distance between the center of the servo track and the data tracks can be identified.
The data storage capacity of the magnetic tape can be increased by increasing the number of data tracks and corresponding servo tracks. However, as the number of data and servo tracks increases, the tracks usually become narrower and more crowded on the surface of the data storage tape. Track width reduction may decrease a signal-to-noise ratio (SNR) of the recording system. The signals detected by the servo controller are proportional to the system SNR, therefore, increasing the storage capacity can cause the strength of servo signals to decrease.